Electrical power tool having a motor control circuit for increasing the effective torque output of the power tool

ABSTRACT

A power tool such as an electric drill typically contains a gear train that couples the output spindle of the motor to the tool bit-receiving chuck and has associated therewith a degree of looseness which must be taken up before torque from the motor is applied to the tool bit. The control circuit for the power tool increases the effective torque output of the tool after a predetermined torque level is attained, by alternately turning the motor on and off, with the duration of the off-time sufficient to permit the gear train to relax, thus giving the motor a &#34;running start&#34; when power is reapplied. Various alternative schemes for transitioning to this ratcheting mode of operation are disclosed including the sensing of a predetermined threshold current, a predetermined increase in motor current, and a predetermined rate of deceleration in motor speed. The control circuit preferably provides the operator with means for adjusting the transition point as well as for varying the duration of the on-time in the ratcheting mode. A corresponding method for controlling the operation of the electric motor of a power tool is also disclosed.

TECHNICAL FIELD

This invention relates to electrically driven power tools and, inparticular, to a power tool such as a variable speed drill having amotor control that is adapted to increase and better control theeffective torque output of the tool.

BACKGROUND OF THE INVENTION

Electrical power tools, such as variable speed drills and powerscrewdrivers, typically include a motor control circuit that is adaptedto control the speed of the output spindle of the tool by controllingthe amount of current supplied to the motor. The desired motor speed isusually selected by the operator by varying the position of the triggerswitch.

If the power tool is provided with an open loop motor control circuit,the speed of the output spindle of the tool will decrease as the tool isloaded and the current drawn by the motor will increase. Accordingly, ifa relatively constant output speed is desired, the operator mustmanually compensate for the reduction in motor speed as the tool isloaded by further retracting the trigger switch to increase the powerapplied to the motor. If the power tool is provided with a closed loopmotor control circuit, the control circuit is typically designed toautomatically increase the amount of power supplied to the motor as theoutput spindle of the tool is loaded in order to maintain the desiredspeed.

Thus, when employed in a power screwdriver to drive a screw into aworkpiece, for example, the current drawn by the motor will increase asthe torque required to drive the screw increases, regardless of whetherthe control circuit provides open or closed loop control. This operationwill continue until either the operator releases the trigger or themotor stalls as the increased torque required to drive the screw exceedsthe torque capacity of the tool. Consequently, the effectiveness of manyportable power tools, particularly power screw-drivers, is directlyrelated to the tool's maximum torque output capacity. Obviously, thegreater the output capacity of the tool, the more useful and versatilethe tool. However, in order to significantly increase the torque outputcapacity of a tool, it is generally regarded as being necessary(assuming changes to the gear train are not an option) to increase thesize of the motor and, consequently, the size, weight, and cost of thetool.

Accordingly, it is the primary object of the present invention toprovide a portable electrical power tool having a motor speed controlcircuit that is able to substantially increase the effective torqueoutput of a power tool for a given size motor and gear train.

In addition, it is an object of the present invention to provide aportable electric power tool, such as a power screwdriver having a motorcontrol circuit, that enables the operator to better control the torqueoutput of the tool, which is particularly beneficial when driving ascrew into a workpiece.

The motor control circuit employed in the present invention is able toachieve these objectives by intermittently pulsing the motor fully onand then fully off for predetermined periods of time after a thresholdcurrent level is attained. More specifically, it has been found that ifthe motor of a power drill is turned completely off for a length of timesufficient to allow the gear train coupled to the motor to at leastpartially "relax", and then full voltage is reapplied to the motor, themotor is able to build up potential energy before the looseness (i.e.,backlash) is removed in the gear train. In effect, the motor is affordeda "running start" while the gear train is relaxed. When the backlash inthe gear train is removed, the sudden impact of the motor torque on thegear train causes a sudden and high burst of torque to be imparted tothe output spindle of the drill, and hence to the driving bit securedthereto. When this "full on" and "full off" control of the motor isalternately repeated, the motor is able to provide a series of bursts oftorque to the gear train which in turn can be used to better finishdriving a wood screw into and below the surface of a workpiece. Thepresent control scheme thus provides better user control due to the factthat the screw does not rotate too much when static friction isovercome. Rather, with each torque pulse, static friction is overcomeand the screw is incremented a fraction of a turn.

While the alternately full-on and full-off operation described above hasbeen found to be particularly helpful and effective when used to drivewood screws and other like implements into a work surface, it has alsobeen found to be an effective means for "breaking loose" a screw or likefastener which is tightly seated in a workpiece, where other forms ofpower tools such as conventional variable speed drills are unable to doso. By reversing the action of the variable speed drill and applying thealternating full-on and full-off operation described above, the burstsof high torque applied by the motor have been found to be extremelyeffective in overcoming the high level of stiction force required toinitiate removal of such fasteners.

Accordingly, it is a further object of the present invention to providean electrically driven power tool, such as a variable speed power drill,which incorporates a control circuit for controlling a motor thereofsuch that the motor can be alternately pulsed fully on and then fullyoff at a predetermined cycle time during operation of the drill.

It is another object of the present invention to provide an electricallydriven power tool having such a control circuit that further provides anoperator of the power tool with a means for adjusting the point at whichthe alternating full-on and full-off operation is initiated.

It is yet another object of the present invention to provide anelectrically driven power tool which automatically enters thealternating full-on and full-off mode of operation when the currentthrough the motor exceeds an operator adjustable threshold levelsetting.

Finally, it is an alternative object of the present invention to providean electrically driven power tool that provides the operator withcontrol over the magnitude of the torque bursts during the alternatingphase of operation of the tool.

SUMMARY OF THE INVENTION

The above and other objects are provided by a portable electric powertool having an electronic control circuit and method in accordance withpreferred embodiments of the present invention. The control circuit ispreferably disposed within the housing or body of the electricallydriven power tool, which is represented illustratively herein as avariable speed power drill. The control circuit generally comprisesoperator adjustable means for setting a threshold current level whichdefines a transition point at which alternating on and off operation ofthe motor is initiated; a trigger switch for selecting the desired speedof the motor; a current sensing circuit for sensing the current flowingthrough the motor; a switching circuit for controlling the flow ofcurrent to the motor; and a controller for comparing the current sensedby the current sensing circuit relative to the threshold current levelselected by the operator and controlling the switching circuit tocontrol the amount of current applied to the motor. When the currentdrawn by the motor exceeds the selected current threshold level, thecontroller is adapted to temporarily interrupt current flow for apredetermined "off-time" interval, and then reapply a maximum currentsignal for a predetermined on-time interval, and to alternate this onand off operation until the trigger switch is released.

The off-time interval during which the controller causes the switchingcircuit to temporarily interrupt current flow to the motor is sufficientto allow the gear train coupled to the motor of the power tool tosufficiently "relax" before maximum current is reapplied to the motor. Avalue representing this time duration is preferably stored in a memoryof the controller and is unique to the gear train of the particularpower tool being controlled.

By alternately applying a maximum current signal for a desired time andthen interrupting current flow for a predetermined time, the motor ofthe power tool is caused to generate successive "bursts" of torque tothe gear train of the power tool which significantly increases theeffective torque output of the power tool. This technique further hasbeen found to be extremely effective in "breaking loose" tightly setwood screws and the like, which other conventionally controlled powertools having comparable-sized motors are unable to achieve.

In several preferred embodiments of the invention, a memory is includedfor storing a plurality of predetermined "on-times" which the controlleraccesses depending on the setting of the current threshold level settingmeans. Thus, on-times of varying duration can be selected by thecontroller to precisely meet the anticipated conditions of a specificapplication.

In an alternative preferred embodiment of the present invention, thecurrent comparison performed by the controller is modified in accordancewith the changing (i.e., increasing) speed of the motor as the triggerswitch is squeezed during operation of the power tool to increase motorspeed. In this instance the threshold current level signal selected bythe operator is decreased as the speed of the motor increases. With thisembodiment a speed sensor is employed to monitor the speed of the motorand provide a signal representative thereof to the controller. As thespeed of the motor increases due to progressive engagement of a triggerof the power tool, the controller decrements the operator-selectedthreshold current level signal. This alternative embodiment furtherhelps to compensate for the inertia of the gear train at higher motorspeeds and helps provide even more consistent results independent of themotor speed of the power tool.

In yet another alternative preferred method of operation of the presentinvention, the transition point for beginning the alternating on and offoperation (referred to also as the "ratcheting mode" of operation) ofthe motor is determined in accordance with a predetermined percentageincrease in the sensed motor current. With this method the currentthrough the motor is initially measured. After a predetermined timedelay, a second current measurement is made. This operation is repeatedcontinuously until the second current measurement exceeds the initialcurrent measurement by a predetermined factor. At that point thecontroller initiates the alternating on and off operation of the motor.In this embodiment the operator-adjustable threshold current level meansis replaced by a means for allowing the operator to adjust the desiredon-time of the motor once the ratcheting mode of operation has begun.

This embodiment and method of operation thus provides a method for"automatically" sensing the size of a screw (and thus the torquerequired to drive the screw) as the operator begins driving the screwinto a workpiece, based on the initial current reading. Since thecurrent required to drive a large screw is greater, in the initialstage, than that required for a small screw, setting the transitionpoint in accordance with a predetermined increase in current (e.g., 25%or 50%) automatically serves to adjust the transition point at which theratcheting mode of operation begins in accordance with the size of thescrew being driven.

In yet another alternative preferred mode of operation of the presentinvention, the transition point is determined by a predetermined drop inmotor speed. In this embodiment, the ratcheting mode of operation of themotor is initiated when the motor speed drops below a predeterminedspeed, or by a predetermined amount (i.e., percentage), or by apredetermined rate.

In a further alternative embodiment, it has been determined to beadvantageous to provide the operator with control over the magnitude ofthe torque bursts during the ratcheting mode of operation. In otherwords, rather than providing fixed on-time/off-time periods during theratcheting mode of operation, it may also be desirable to provide theoperator with the ability to continue to vary the duty cycle of thevoltage signal during the ratcheting mode of operation in accordancewith the position of the trigger switch.

In particular, conventional variable speed power tools control the speedof the motor by varying the duty cycle of the voltage signal supplied tothe motor. The frequency of the duty cycle signal is set sufficientlyhigh--typically 2 KHz to 12 KHz--so that the motor operates smoothlyeven though the power is actually being rapidly cycled on and off. Thepercentage on-time of the duty cycle signal, and hence the average powerlevel, supplied to the motor is controlled by the operator in accordancewith the position of the trigger switch.

Consequently, it will be appreciated that the transition from normalvariable speed control of the motor to the above-describedratcheting-mode of operation can be viewed simply as a change in thefrequency of the duty cycle control signal. In other words, theratcheting mode of operation can be achieved simply by switching from arelatively high frequency control signal to a relatively low frequencycontrol signal (e.g., 10-50 Hz), the period of which is greater than theresponse time of the motor. Considered in this manner, it is readilyapparent that it is possible to continue to provide trigger switchcontrol over the duty cycle of the control signal during the ratchetingor low frequency mode of operation, and thereby provide the operatorwith the ability to control the magnitude of the torque bursts. This, inturn, provides the operator with greater control when seating a screwinto a workpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

The various advantages of the present invention will become apparent toone skilled in the art by reading the following specification andsubjoined claims and by referencing the following drawings in which:

FIG. 1 is an elevational side view of a typical variable speed powerdrill with which the control circuit of the present invention may beused;

FIG. 2 is a simplified block diagram of a preferred embodiment of thecontrol circuit of the present invention;

FIG. 3 is a flowchart of the steps of operation performed by the controlcircuit in implementing the alternating on and off, or "ratcheting" orlow frequency, mode of operation;

FIG. 4 is a graph of the current flow through the motor verses the depthof a screw being driven in by a power tool incorporating the controlcircuit of the present invention while a trigger of the power tool isheld steady in an engaged position;

FIG. 5 is a flowchart of an alternative method of control fordetermining the transition point as to when the ratcheting mode ofoperation is to begin;

FIG. 6 is a flowchart of another alternative method of control in whichthe current through the motor of the power tool is measured repeatedlyand the ratcheting mode is implemented when the current increases by apredetermined factor from an initial measurement;

FIG. 7 is a flowchart of another alternative preferred method of controlfor determining an appropriate transition point by sensing for apredetermined drop in motor speed; and

FIG. 8 is a partial flowchart diagram of an alternative manner ofcontrolling the power tool during the ratcheting or low frequency modeof operation illustrated in FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, an electrically driven power tool in the form of acordless (i.e., battery driven) variable speed power drill 10incorporating an electronic control circuit 12 in accordance withpreferred embodiments of the present invention is illustrated. As willreadily be appreciated by those skilled in the art, the motor controlcircuit taught by the present invention is adaptable to other types ofelectrical power tools, such as power screwdrivers, electric pop rivetguns, and the like. The control circuit 12 includes a current thresholdpotentiometer 14 which is disposed in a convenient position on the drill10 to allow an operator to conveniently adjust the potentiometer 14 asneeded. The function of this potentiometer 14 will subsequently bedescribed in greater detail.

Optionally included is an on-time adjustment potentiometer 15 which maybe used in lieu of potentiometer 14 in an alternative preferredembodiment of the present invention. The on-time potentiometer 15provides an operator with direct control over the on-time intervalimplemented during the "ratcheting" mode of operation of the motor 16.It will be appreciated, however, that potentiometers 14 and 15 couldeasily be used simultaneously to provide the operator with control overthe point at which the ratcheting mode of operation begins as well asthe duration of the on-time, if such is desired.

The drill 10 includes, in conventional fashion, a motor 16 and arechargeable battery 18 for powering the motor 16. While the drill 10has been illustrated as battery powered, it will be appreciated that thecontrol circuit 12 of the present invention could just as easily be usedwith an A/C powered drill with little or no modification, providedsuitable phase control circuitry is included.

The motor 16 of the drill is adapted to drive through a conventionalgear train 20, a tool bit-receiving chuck 22. A trigger switch 24controls the battery voltage across the motor 16 and therefore thecurrent flowing through the motor 16 to provide an operator with thecapability of varying the speed of the chuck 22 to suit various workneeds.

With reference to FIG. 2, a block diagram of a preferred embodiment ofthe control circuit 12 is illustrated. The control circuit 12 is used toimplement the alternating on and off operation of the motor 16, whichwill hereafter be referred to as the "ratcheting" mode or low frequencymode of operation.

The control circuit 12 generally includes a controller 26 in the form ofa microcomputer, the threshold current level potentiometer 14, a memorydevice 28 such as a read-only memory (ROM), and a current sensingcircuit 30 coupled in series with the motor 16. A switching circuit inthe form of a metal oxide silicone field effect transistor (MOSFET)drive circuit 32 is provided for controlling the voltage applied acrossthe motor 16 and thus the current flow through the motor 16, inaccordance with the duty cycle of the pulse width modulated ("PWM")control signal received from the microcomputer 26. The on-timeadjustment potentiometer 15 is also shown in phantom indicating itspresence as being optional.

The controller circuit 12 further includes the trigger switch 24 whichprovides a signal to the microcomputer 26 in accordance with the degreeof retraction thereof by the operator. A DC battery pack 18 and aconventional voltage regulator 34 connected across the battery areprovided for supplying a regulated DC voltage across the motor 16 and tothe microcomputer 26.

The control circuit 12 may optionally include a speed sensor 36 forsensing the speed of the motor 16 and providing a signal in accordancetherewith to the microcomputer 26. The speed sensor 34 may take the formof a variety of well-known speed sensing devices such as opticalencoders or Hall-effect sensors which are capable of supplying a seriesof pulses to the microcomputer 26 which are representative of thefrequency of rotation of the motor 16. The use of the speed sensor 34will be described in greater detail hereinafter in connection with analternative preferred embodiment of the present invention.

The microcomputer 26 preferably is comprised of an 8-bit microprocessorwhich includes an on-board memory 29, preferably in the form ofread-only memory (ROM), for storing, inter alia, a constant which in thepreferred embodiment represents an "off-time" duration sufficient toallow the gear train 20 of the drill 10 to completely relax aftercurrent flow to the motor 16 is interrupted. The microcomputer 26 alsoaccesses the memory 28 to read a plurality of values stored in a look-uptable therein which represents varying on-time intervals for leaving themotor 16 fully on when the ratcheting mode is implemented. Finally, themicrocomputer 26 is responsive to the threshold current levelpotentiometer 14 to provide an operator with the capability of adjustingthe point at which the ratcheting mode of operation is to begin. Thispoint will be referred to hereafter as the "transition" point.

It should be appreciated that the relax time, and thus the desiredoff-time, will vary from tool to tool depending on the design of thegear train. Accordingly, power tools such as drills having gear trainsof differing design will most likely require different off-times toallow their gear trains to completely relax. The off-time for a specificgear train design may be determined by any suitable testing procedurewhich provides a relatively accurate determination of the time intervalrequired, once torque is removed from the gear train, for the gear trainto return to a "relaxed" condition. With the preferred embodimentsdescribed herein, an off-time within a range of about 20 ms to 100 msprovides sufficient time for the gear train to completely relax,although this can vary greatly depending upon the particular type oftool.

In addition, it will be appreciated that it is not critical to thepresent invention that the duration of the off-time period be sufficientto enable the gear train of the power tool to completely relax. Inparticular, if the off-time period is such that the gear train isallowed to only partially relax, an enhanced torque effect willnonetheless be realized when power is reapplied, but simply to a lesserdegree than if the gear train is allowed to completely relax.

With reference to FIG. 3, a description of the operation of the controlcircuit 12 will now be provided. Initially, the microcomputer 26 readsthe current threshold level selected by the operator via the currentthreshold potentiometer 14, as indicated at step 38, and waits for theoperator to activate the trigger switch, as indicated at step 40.

If the test at step 40 proves true, the microcomputer 26 controls theMOSFET drive circuit 32 to provide a voltage signal across the motor 16which is proportional to the degree of engagement of the trigger 24, asindicated at step 42. The microcomputer 26 then reads the output fromthe current sensing circuit 30 to determine if the motor current isgreater than the current threshold signal provided by the currentthreshold potentiometer 14, as indicated at step 44. If this test provesfalse, then steps 42 and 44 are repeated until step 44 proves true.

When the test at step 44 proves true, the microcomputer 26 accesses thelook-up table stored in the memory 28, as indicated at step 46, toobtain the appropriate on-time to be used during the ratcheting mode ofoperation. The microcomputer 26 then causes the MOSFET drive circuit 32to interrupt current flow to the motor 16 for the predeterminedoff-time, as indicated at step 48. When the off-time interval hasexpired, the microcomputer 26 causes the MOSFET drive circuit 32 toreapply a maximum current flow to the motor 16 for the predeterminedon-time interval, as indicated at step 50. As discussed previouslyherein, the off-time interval is preferably of a duration sufficient toallow the gear train 20 to completely relax.

When the on-time interval has expired, the microcomputer 26 again checksto determine if the trigger 24 is still engaged, as indicated at step52. If this test proves true, then steps 48-52 are repeated until thetest at step 52 proves false. When the test at step 52 proves false,indicating that the work operation is complete, the power to the motoris disconnected and the program returns to the start.

In the preferred embodiments of the present invention, the maximumcurrent signal is a current signal which is sufficiently large to drivethe motor at or near its maximum rated speed. This current signal isfurther applied and removed in a rapid, pulse-like fashion such that themotor 16 "sees" virtually instantaneous "turn-on" and "turn-off"signals.

Optionally, a plurality of varying on-times may be stored in the memory28 to enable the length of time during which the maximum current signalis applied to be correlated more precisely to the setting of the currentthreshold potentiometer 14. For example, if the transition point is setto occur at about 80% of maximum rated current flow, then a shorteron-time may be desirable than that required if the transition point isset to 90% of maximum rated current flow. Thus, by varying the on-timeinterval in accordance with the current threshold potentiometer 14, theduration of the on-time can be chosen by the microcomputer to maximizethe torque producing capability of the motor 16 to suit the needs ofspecific applications.

Referring to FIG. 4, a graph shows an exemplary current flow through themotor 16 (and thus the torque generated by the motor 16) as regulated bythe control circuit 12 when installing a wood screw completely into apiece of wood. Initially, the current flow through the motor 16 issubstantially continuous, as represented by curve 54. In actuality, dueto the relatively high frequency of the PWM control signal, there existsa corresponding high frequency ripple in the motor current during thismode of operation. When the motor current exceeds the threshold currentlevel 56 set via the current threshold potentiometer 14, whichrepresents the transition point 58, the ratcheting or low frequency modeis initiated. Current flow to the motor 16 is quickly interrupted forthe predetermined off-time 60 to allow the gear train 20 in thepreferred embodiment to completely relax. After this time interval hasexpired, the microcomputer 26 causes the MOSFET drive circuit 32 torapidly apply maximum current flow to the motor 16. This maximum currentflow is maintained for the on-time 62 read from the look-up table in thememory 28. The cycle is then repeated until such time as themicrocomputer 26 detects that the trigger switch 24 of the drill 10 hasbeen released, as indicated by portion 64 of the waveform.

Importantly, it will be appreciated, for purposes of the scope of thepresent invention as described herein and as claimed, that it isirrelevant whether the ratcheting mode of operation is initiated by an"off" time period, as shown in FIG. 4, or an "on" time period.Consequently, when the control circuit is described herein as initiallyinterrupting power to the motor in response to the detection of thetransition point, it is to be understood that the control circuit couldjust as readily initially apply full power to the motor in response tothe detection of the transition point.

Alternatively, it is readily possible to modify the software algorithmillustrated in FIG. 3 so that the operator of the power tool is able tocontrol the magnitude of the torque bursts during the low frequency orratcheting mode of operation. In particular, rather than providing fixedon-time and off-time periods as shown in FIG. 3, the microcomputer 26can be programmed to merely reduce to a relatively low level thefrequency of the PWM control signal and continue to set the motorvoltage proportional to the position of the trigger switch. Thisalternative control scheme is illustrated in FIG. 8. In this embodiment,during the low frequency mode, the percentage on-time of the duty cyclesignal, and hence the average motor voltage signal, supplied to themotor is set in accordance with the position of the trigger switch 24.Thus, following detection of the transition point, the microcomputer 26reduces the frequency of the PWM control signal at step 45 to apredetermined relatively low level, typically between 10-50 Hz. Thisfrequency level is selected to be sufficiently low such that the periodof the PWM control signal is substantially greater than the responsetime of the motor 16. In particular, the period of each cycle of the PWMcontrol signal during the low frequency mode of operation must besufficiently greater than the response time of the motor to enable thegear train of the power tool to at least partially relax during theoff-time portion of the cycle, which, of course, will be something lessthan the total time period of each cycle, depending on the position ofthe trigger switch. In other words, at step 47 the microcomputer isprogrammed in this embodiment to set the percentage duty cycle of thePWM control signal in accordance with the position of the triggerswitch. This will produce corresponding on-time and off-time periodswhich, added together, equal the period of one cycle of the PWM controlsignal. Consequently, the duration of the off-time portion of each cyclemust be long enough, at least at moderate trigger switch settings, topermit at least partial relaxation of the gear train for the advantagesof the present invention to be realized. In a typical variable speeddrill, a frequency of 10-50 Hz has been found to be acceptable, althoughthis too can vary depending upon the characteristics of a particulartool.

In this embodiment, therefore, the operator is able to control themagnitude of the torque bursts, and thus control the rate at which ascrew is seated into a workpiece, by varying the position of the triggerswitch. Accordingly, the operator can, for example, achieve a quarterturn or a half turn of the screw with each torque burst depending uponthe position of the trigger switch, and thus properly seat a screw intoa workpiece in a very controlled manner. Consequently, the presentinvention avoids the dilemma of risking the over-application of a largeburst of power to finish setting a screw and inadvertently causing thescrew to be set too deeply below the surface of the workpiece.

Referring now to FIGS. 5-7, various alternative methods for determiningthe appropriate time to transition from the high frequency mode to theratcheting or low frequency mode of operation are disclosed. Withinitial reference to FIG. 5, the control circuit 12 in this embodimentnot only monitors the current flowing through the motor 16 to detectwhen the transition point has occurred, but also incorporates the use ofthe speed sensor 36 (shown in FIG. 2) to modify the threshold currentlevel signal provided by the current threshold potentiometer 14 as setby the operator. In particular, the suitability of a particular currentthreshold is dependent upon the speed of the motor when the threshold isattained. In other words, as motor speed is increased, the amount ofinertia in the system increases which will cause a screw to continue toturn after the motor has been turned off. Consequently, in order toprovide consistent results, it is preferable to adjust the currentthreshold in accordance with the speed of the motor at a particularpoint during the screw setting process that is relatable to theprojected speed of the motor when the threshold is attained.

As set forth in the flowchart diagram, the microcomputer 26 initiallyreads the current threshold potentiometer 14, as indicated at step 66,and waits for the operator to actuate the trigger switch 24, asindicated at step 68. Once the microcomputer 26 detects that the trigger24 has been pulled, the appropriate motor voltage is set proportional tothe trigger setting, as indicated at step 70.

The microcomputer then waits a predetermined time period, as indicatedat decision step 72. Once this time period has elapsed, the speed of themotor (V) is read at step 76, and then the current threshold level isadjusted based upon the actual speed of the motor (V) at this point andthe setting of the current threshold potentiometer 14, as indicated atstep 78. Next, a flag is set (step 79) so that the adjustment process isnot repeated (decision step 74) and the program continues in the loopuntil the motor current exceeds the adjusted current threshold level 80.

If the test at step 80 proves true, then the microcomputer 26 accessesthe look-up table in the memory 28 to determine the appropriate on-time,as indicated at step 82, to be applied during the ratcheting mode ofoperation. The microcomputer 26 then begins the ratcheting mode byturning the motor 16 full-off for the off-time, as indicated at step 84,and then turning the motor 16 full-on for the selected on-time, asindicated at step 86. Another check is then made to determine if thetrigger 24 is still being pulled by the operator, as indicated at step88. If this test proves true, then steps 84, 86, and 88 are repeateduntil the test at step 88 proves false, whereupon power to the motor isterminated.

The alternative method of control set forth in FIG. 5 thus provides ameans by which the transition point can be modified proportionally withdifferences in motor speed. This allows the control circuit 12 tocompensate for the inertia generated at high motor speeds whichcontinues to apply torque to the screw after the motor 16 is turned off.Accordingly, this method can provide even more consistent results indetermining the most effective transition point independent of how fastthe motor 16 is being operated.

Referring now to FIG. 6, another alternative method of control fordetermining the appropriate transition point is set forth. This methodessentially involves monitoring the current flow through the motor 16 todetermine when the current flow has increased by a predetermined factor(for example, doubled or tripled), for signalling the microcomputer 26to implement the ratcheting mode of operation. With this method ofcontrol the optional on-time potentiometer 15 (FIGS. 1 and 2) may beincorporated to provide direct operator control over the on-timeinterval during the ratcheting mode of operation in lieu of the currentthreshold potentiometer 14.

Once the operator has actuated the trigger switch (step 92), the on-timepotentiometer 15 is read, as indicated at step 90, and the motor voltageis set proportional to the setting of the trigger 24, as indicated atstep 94. After a first predetermined time interval (T1), a first currentflow reading I₁ through the motor 16 is taken, as indicated at-step 96.The microcomputer 26 then waits a second predetermined time interval(T2), as indicated at step 98, before taking a second current reading I₂through the motor 16, as indicated at step 100. A test is then made bythe microcomputer 26 to determine if I₂ is greater than I₁ by apredetermined factor, as indicated at step 102. If this test provesfalse, the microcomputer 26 checks to see if the trigger 24 is stillpulled, as indicated at step 104 and, if so, repeats steps 98, 100, and102 until the test at step 102 proves true.

When the test at step 102 proves true, the microcomputer 26 accesses thememory 28 to obtain the appropriate on-time value from the look-uptable, as indicated at step 106. The microcomputer 26 then controls theMOSFET drive circuit 32 to interrupt current flow to the motor 16, asindicated at step 108, thus initiating the ratcheting mode of operation.

The current flow is interrupted for the preset off-time duration, afterwhich a maximum current flow signal is applied to the motor 16 for theselected on-time, as indicated at step 110. The ratcheting mode ofoperation is repeated until the microcomputer 26 detects that thetrigger switch 24 has been released, as indicated at step 112, whereuponthe power to the motor is terminated.

The alternative method of control described above in connection withFIG. 6 provides a method for determining the transition point which also"automatically" senses the size of the screw being installed based onthe first current reading at step 96. Accordingly, this method has theadvantage of automatically tailoring the transition point to occur at anappropriate time to accommodate different size wood screws. As will bereadily appreciated by those skilled in the art, the program illustratedin FIG. 6 can also be readily modified to detect a predeterminedpercentage increase in motor current or a predetermined rate of increasein motor current as the transition event before switching to the ratchetmode of operation.

Yet another alternative method of determining the transition point isshown in connection with FIG. 7. The steps shown in FIG. 7 may beimplemented in lieu of steps 96-102 of the method described inconnection with FIG. 6. With reference to FIG. 7, after steps 90-95 ofFIG. 6 have been performed, the motor 16 speed V₁ is read as indicatedat step 114. The microcomputer 26 then waits a predetermined timeinterval (T2), as indicated at step 116, before again reading the motor16 speed V₂, as indicated at step 118. A test is then made to determineif the motor 16 speed has decreased a predetermined amount (for example,by 50 percent), as indicated at step 120. If this test proves false, acheck is made to determine if the trigger 24 is still being pulled, asindicated at step 122. If this test proves false, the method loops backto the very start of the program as indicated in FIG. 6.

If the test at step 122 proves true, then steps 116 through 120 arerepeated. Once the test at step 120 proves true (i.e., the motor 16speed has decreased by a predetermined amount) the ratcheting mode ofoperation is implemented in accordance with steps 106-110 of FIG. 6.Optionally, of course, the test at step 120 could be modified to detecta predetermined percentage drop in motor speed or a predetermined rateof deceleration.

By the method described in connection with FIG. 7, a relatively simplesequence of operation is provided for determining an appropriatetransition point at which the ratcheting mode of operation is to occurwhich also takes into account the size of the wood screw being driven,as well as the hardness of the wood into which the wood screw is beingdriven. By sensing for a predetermined amount or percentage drop inmotor speed, the ratcheting mode can be implemented at appropriate timesfor a variety of applications to optimize the effectiveness of the tool.

The method of FIG. 7 also does not require the use of the currentthreshold potentiometer 14. Moreover, the on-time potentiometer 15 isalso not required for this preferred method of control.

With all of the embodiments described herein, it will be appreciatedthat the ratcheting or low frequency mode of operation could easily beemployed, with little or no modification, to break loose tightly seatedwood screws, nuts, etc. where the continuous application of torqueproves ineffective. It will also be appreciated that the ratcheting orlow frequency mode of operation disclosed herein could also be adaptedwith little or no modification for a variety of power tools including,but not limited to, power rivet tools, for example, to help unjam suchtools as needed.

Those skilled in the art can now appreciate from the foregoingdescription that the broad teachings of the present invention can beimplemented in a variety of forms. Therefore, while this invention hasbeen described in connection with particular examples thereof, the truescope of the invention should not be so limited since othermodifications will become apparent to the skilled practitioner upon astudy of the drawings, specification, and following claims.

What is claimed is:
 1. A power tool having a tool holder, gear meanscoupled to said tool holder, operator actuable input means, and anelectric motor for driving said tool holder via said gear means, saidgear means having associated therewith a degree of looseness that mustbe taken up before torque from said motor is transferred to said toolholder and which further results in said gear means relaxing after powerto said motor is interrupted; the improvement comprising:current sensingmeans for sensing a current flow through the motor of the power tool;and control means responsive to actuation of said input means forcontrolling the power supplied to the motor, and further responsive tosaid current sensing means for detecting when the current through themotor has exceeded a predetermined current level and thereupon, withsaid input means actuated, repetitively interrupting and reapplyingpower to the motor such that the off-time intervals are of sufficientduration to enable the gear means to at least partially relax.
 2. Thepower tool of claim 1 further comprising operator adjustable means incommunication with said control means for adjusting said predeterminedcurrent level.
 3. The power tool of claim 1 wherein said control meansincludes memory means for storing a plurality of selectable off-timeintervals.
 4. The power tool of claim 1 wherein said off-time intervalis sufficient to permit said gear means of said power tool to completelyrelax after current through said motor is completely interrupted.
 5. Anelectrical power tool having a control circuit for controlling the motorof the electrical power tool such that said motor alternately developsmaximum and minimum levels of torque when a predetermined loading uponsaid motor is reached during operation of said power tool, said controlcircuit comprising:means for sensing a current flow through said motor;switching means for controlling said current flow through said motor;controller means responsive to said current sensing means forcontrolling said switching means to thereby control said current flowthrough said motor; means for providing an operator-adjustable currentthreshold signal to said controller means; memory means in communicationwith said controller means for storing information corresponding to aplurality of predetermined on-time intervals; such that when saidcurrent flow detected by said current sensing means exceeds said currentthreshold signal said controller means causes said switching means tocyclically interrupt current flow through said motor for a time intervalin accordance with a predetermined off-time interval sufficient to allowa gear train associated with said motor to at least partially relax, andapply a maximum current signal to said motor in accordance with aselected one of said predetermined on-time intervals.
 6. The power toolof claim 5 further comprising speed sensor means for monitoring thespeed of said motor and providing a signal in accordance therewith tosaid controller means.
 7. The power tool of claim 6 wherein saidcontroller means modifies said current threshold signal in accordancewith said signal received from said speed sensing means.
 8. A method forcontrolling a motor of an electric power tool such that said motor iscaused to repetitively generate substantial variations in torque outputonce a predetermined current flow through said motor is reached, saidmethod comprising the steps of:applying power to the motor in accordancewith an operator actuable switch means; providing a current thresholdlevel; sensing the current flow through said motor of said power tool;comparing said sensed current flow through said motor against saidcurrent threshold level; and with said switch means actuated,repetitively cycling the motor on and off when said current flow throughsaid motor exceeds said current threshold level such that current flowthrough said motor is alternatively turned off for a predeterminedoff-time sufficient to allow a gear train associated with said motor toat least partially relax, and after said predetermined off-time hasexpired, current flow is re-established through said motor for apredetermined on-time interval.
 9. The method of claim 8 wherein saidoperator actuable switch meanscomprises an operator-controllable triggerswitch for controlling the speed of said motor; and further comprisingthe steps of: sensing the speed of said motor; and vary said currentthreshold level in accordance with said sensed motor speed.
 10. A methodfor controlling a motor of an electrically driven power tool such thatsaid motor is caused to be driven alternately full on and off duringoperation of said power tool, the method comprising the stepsof:providing a motor voltage in accordance with the position of anoperator-actuable trigger switch; taking a first reading of the current(I₁) through said motor; waiting a predetermined time interval; taking asecond current reading (I₂) through said motor; comparing I₂ with I₁ todetermine if I₂ is greater than I₁ by at least a predetermined factor;when I₂ is greater than I₁ by at least said predetermined factor,interrupting current flow to said motor to thereby turn said motorfull-off for a predetermined off-time duration, where said off-timeduration is sufficient to allow a gear train of said power tool to atleast partially relax, and after said off-time duration has expired,providing a maximum current flow to said motor to thereby turn saidmotor full-on for a predetermined on-time; when said on-time durationhas expired, determining whether said trigger switch is still beingactuated; and if said trigger is still being actuated, repeating saidsteps of alternately interrupting current flow and applying a maximumcurrent flow until said trigger is released by the operator.
 11. Amethod for controlling a motor of an electrically driven power tool suchthat said motor is caused to be driven alternately full on and offduring operation of said power tool, the method comprising the stepsof:providing a motor voltage in accordance with the position of anoperator-actuable trigger; taking a first reading of the speed of saidmotor; waiting a predetermined time interval; taking a second reading ofthe speed of said motor; when said second reading has decreased inmagnitude from said first reading by a predetermined amount,interrupting current flow to said motor for an off-time sufficient toallow a gear train associated with said motor to at least partiallyrelax; after said off-time has expired, applying a maximum currentsignal to said motor for a predetermined on-time; determining if saidtrigger switch is still actuated; and if said trigger switch is stillactuated, repeating said steps of alternately interrupting current flowand applying a maximum current flow until said trigger switch isreleased by the operator.
 12. A power tool having a tool holder, gearmeans coupled to said tool holder, and an electric motor for drivingsaid tool holder via said gear means, said gear means having associatedtherewith a degree of looseness which must be taken up before torquefrom said motor is transferred to said tool holder and which furtherresults in said gear means relaxing after power to said motor isinterrupted; the improvement comprising a control circuit forcontrolling the application of power to said motor including sensingmeans for sensing an operating parameter of said motor, and controllermeans responsive to said sensing means for detecting a predeterminedoccurrence in said operating parameter and thereupon cyclicallyinterrupting power to said motor for an off-time sufficient to enablesaid gear means to at least partially relax and reapplying power to saidmotor for an on-time to enable said motor to build up potential energyas the looseness in said gear means is taken up to thereby causesubstantial repetitive variations in the torque applied to the toolholder.
 13. The power tool of claim 12 wherein said sensing meanscomprises a current sensor for sensing the current being drawn by saidmotor and said controller means is adapted to detect a predeterminedchange in said motor current.
 14. The power tool of claim 13 whereinsaid controller means is adapted to detect when said motor currentexceeds a predetermined level.
 15. The power tool of claim 14 whereinsaid control circuit further includes adjustment means for varying saidpredetermined current level.
 16. The power tool of claim 15 wherein saidcontroller means is adapted to automatically vary said on-time inaccordance with the setting of said predetermined current level.
 17. Thepower tool of claim 14 further including a speed sensor for sensing therotational speed of said motor, and further wherein said controllermeans is further adapted to vary said predetermined current level inaccordance with the sensed rotational speed of the motor.
 18. The powertool of claim 13 wherein said controller means is adapted to detect whensaid motor current has increased by a predetermined amount.
 19. Thepower tool of claim 13 wherein said controller means is adapted todetect when said motor current has increased by a predeterminedpercentage amount.
 20. The power tool of claim 13 wherein saidcontroller means is adapted to detect when the rate of increase in motorcurrent exceeds a predetermined level.
 21. The power tool of claim 12wherein said sensing means comprises a speed sensor for sensing therotational speed of the motor and said controller means is adapted todetect a predetermined change in motor speed.
 22. The power tool ofclaim 21 wherein said controller is adapted to detect a predetermineddecrease in motor speed.
 23. The power tool of claim 21 wherein saidcontroller is adapted to detect a predetermined percentage decrease inmotor speed.
 24. The power tool of claim 21 wherein said controller isadapted to detect a predetermined rate of deceleration in motor speed.25. The power tool of claim 12 wherein said control circuit furtherincludes adjustment means for varying said on-time.
 26. A method ofcontrolling a power tool having a tool holder, gear means coupled tosaid tool holder, and an electric motor for driving said tool holder viasaid gear means, said gear means having associated therewith a degree oflooseness which must be taken up before torque from said motor istransferred to said tool holder and which further causes said gear meansto relax after power to said motor is interrupted; the method comprisingthe steps of:applying power to said motor, monitoring an operatingparameter of said motor, detecting a predetermined occurrence in saidoperating parameter, and thereupon cyclically interrupting power to saidmotor for an off-time sufficient to enable said gear means to at leastpartially relax and reapplying power to said motor for an on-time toenable said motor to build up potential energy as the looseness in saidgear means is taken up to thereby increase the effective torque outputof said motor.
 27. The method of claim 26 wherein said monitoring stepcomprises sensing the current being drawn by said motor and saiddetecting step comprises detecting a predetermined change in said motorcurrent.
 28. The method of claim 27 wherein said detecting stepcomprises detecting when said motor current exceeds a predeterminedlevel.
 29. The method of claim 28 wherein said detecting step comprisesdetecting when the rate of increase in motor current exceeds apredetermined level.
 30. The method of claim 28 further including thestep of selectively setting said predetermined current level.
 31. Themethod of claim 30 further including the step of automatically adjustingsaid on-time in accordance with the setting of said predeterminedcurrent level.
 32. The method of claim 28 further including the steps ofsensing the rotational speed of the motor and adjusting saidpredetermined current level in accordance with said sensed speed. 33.The method of claim 27 wherein said detecting step comprises detectingwhen said motor current has increased by a predetermined amount.
 34. Themethod of claim 27 wherein said detecting step comprises detecting whensaid motor current has increased by a predetermined percentage amount.35. The method of claim 26 wherein said monitoring step comprisessensing the rotational speed of the motor and said detecting stepcomprises detecting a predetermined change in motor speed.
 36. Themethod of claim 35 wherein said detecting step comprises detecting apredetermined decrease in motor speed.
 37. The method of claim 35wherein said detecting step comprises detecting a predeterminedpercentage decrease in motor speed.
 38. The method of claim 35 whereinsaid detecting step comprises detecting a predetermined rate ofdeceleration in motor speed.